Polypropylene (PP)-based resins are generally inexpensive and have excellent mechanical strength, thermostability, chemical resistance, molding workability and recycling properties, and are therefore used in a wide range of fields including industrial materials, automobile parts, household electrical appliances and packaging materials. In recent years it has become a goal to provide products exhibiting a high level of performance in terms of flame retardance, gas barrier properties, thermostability and rigidity that cannot be realized by PP alone, by using combinations thereof with other materials. However, since PP does not have polar groups in the molecule, it exhibits very low adhesive strength for other materials such as other types of resins or metal, wood and the like, and in some cases it fails to adhere at all. One well-known means for solving this problem is a technique of blending crystalline polypropylene with maleic acid and a radical initiator for graft modification, as described in Japanese Examined Patent Publication No. 42-10757, for example. However, the molecular chains of crystalline polypropylene are easily cleaved by the radical initiator, such that the flow property is notably increased while the adhesion interface becomes rigid, and therefore this technique is poorly applicable for uses that require flexibility. Flexible adhesives or compatibilizers have also been proposed which are resin materials obtained by modifying ethylene/α-olefin copolymer rubber with unsaturated carboxylic acids, as disclosed in Japanese Unexamined Patent Publication No. 52-49289, but because such materials are composed primarily of ethylene, they have insufficient heat resistance and compatibility with PP. In addition, Japanese Unexamined Patent Publication No. 55-50040, Japanese Unexamined Patent Publication SHO No. 56-159239 and Japanese Unexamined Patent Publication No. 09-235319 propose materials obtained by modifying PP and rubber blends with maleic acid, but these materials are still problematic because of inadequate tensile strength or adhesive force with the filler due to a lack of dispersibility of the rubber.